.TH LFS-MIGRATE 1 2021-11-08 "Lustre" "Lustre Utilities"
.SH NAME
lfs-migrate \- migrate files or directories between MDTs or OSTs.
.SH SYNOPSIS
.B lfs migrate
.RB [ -h "] [" -v ]
.RI [ SETSTRIPE_OPTIONS " ... ]"
.IR FILE " ..."
.br
.B lfs migrate -m \fISTART_MDT_INDEX
.RB [ -cdHv ]
.I DIRECTORY
.br
.SH DESCRIPTION
Migrate OST objects between OSTs for the specified
.IR FILE ,
or recursively migrate
.I DIRECTORY
and all inodes/directories therein between MDTs.
.SH OST MIGRATE OPTIONS
.P
The
.B lfs migrate
command can be used for moving files from one (or more) OSTs to other
OSTs (e.g. for space balancing between OSTs, or to evacuate an OST for
hardware reasons), to change the stripe count or other layout parameters
of a file (e.g. to increase the bandwidth of a file by striping it over
multiple OSTs), or to move the file between different classes of storage
(e.g. SSD vs. HDD OSTs, or local vs. remote OSTs in different pools).
.P
In OST object migration mode, the command supports the same
.I SETSTRIPE_OPTIONS
listed in
.BR lfs-setstripe (1)
to specify the layout of the target file.  The migrate command differs from
.B lfs setstripe
in that
.B lfs migrate
will copy the data from the existing file(s) using the new layout parameters
to the new OST(s). In contrast,
.B lfs setstripe
is used for creating new (empty) files with the specified layout.
For OST object migration, there additional options available:
.TP
.BR -b , --block
Block access to the file by other applications during data migration
(default).  This prevents other processes from accessing the file during
migration, which prevents data data writes to the old file objects from
being lost.  This should be used if an OST needs to be completely emptied
prior to its removal, to ensure all requested files are migrated off the
OST.
.TP
.BR -h , --help
Print usage message.
.TP
.BR -n , --non-block
Abort migration if concurrent file access is detected.  This can be
used with OST space balancing migration to avoid interfering with file
access by applications if there is not a requirement to migrate any
particular file to the new layout.
.TP
.BR -D , --non-direct
Do
.B not
use
.B O_DIRECT
read and write operations when migrating a file.  The
.B O_DIRECT
option avoids data copy from kernel buffers into userspace, which can
impose CPU and memory overhead on the copy operation, but makes read and
write operations synchronous.  Using the
.B --non-direct
option uses buffered read/write operations, which may improve migration
speed at the cost of more CPU and memory overhead.
.br
This option cannot be used on encrypted files when the encryption key is not
available. It will result in
.B
-ENOKEY.
.TP
.BR -W , --bandwidth
Limit how much file system bandwidth a migrate job can consume.
.TP
.BR --stats
Output verbose stats about migrate job progress in YAML format.
.TP
.BR --stats-interval
Controls how often the stats are output; this defaults to 5 seconds.
.TP
.BR -v , --verbose
Print each filename as it is migrated.
.P
NOTE:
.B lfs migrate
has a complementary
.B lfs_migrate
script which is used to provide extra functionality when migrating file
data between OSTs and has a separate man page.  See
.BR lfs_migrate (1)
for details.
.SH MDT MIGRATE OPTIONS
.TP
.BR -m , --mdt-index=\fIMDT_INDEX [, \fIMDT_INDEX ,...]
The specified
.I DIRECTORY
.B and all subdirectories and inodes
will be migrated to the MDT with the specified
.IR MDT_INDEX .
This is useful if new MDTs have been added to a filesystem and existing user or
project directories should be migrated off old MDTs to balance the space usage
and future metadata workload. If
.I MDT_INDEX
is -1, the MDT index will be balanced by free space and inodes among
available MDTs.  If multiple
.I MDT_INDEX
values are specified in a comma-seperated list, then all
subdirectories will be
.B striped
across all of the specified MDT indices as if an equivalent
.BI -c N
option were given.
.TP
.BR -c , --mdt-count=\fICOUNT\fR
All directories and subdirectories in the tree will be striped across
.I COUNT
MDTs, always using
.I MDT_INDEX
as the primary MDT for the directory.  If
.I MDT_INDEX is
.B -1
then
.I COUNT
directory stripes will be chosen from MDTs proportional to the amount
of free inodes and space on each MDT.  If multiple
.I MDT_INDEX
values are specified in a comma-separated list, then the number of specified
.I MDT_INDEX
values must match
.IR COUNT .
.TP
.BR -d , --directory
Only migrate the specified \fIDIRECTORY\fR and the non-directory inodes that are
directly located within it.
Similar to '\fBls -d\fR' and '\fBlfs getstripe -d\fR'.
.TP
.BR -H , --mdt-hash=\fIHASH_TYPE\fR
Use
.I HASH_TYPE
for the new directory layout.
.RS 1.2i
.TP
.B all_char (type 1)
Sum of ASCII characters modulo number of MDTs. This
provides weak hashing of the filename, and is suitable
for only testing or when the input is known to have
perfectly uniform distribution (e.g. sequential numbers).
.TP
.B fnv_1a_64 (type 2)
Fowler-Noll-Vo (FNV-1a) hash algorithm.  This provides
reasonably uniform, but not cryptographically strong,
hashing of the filename. (default)
.TP
.B crush (type 3)
CRUSH hash algorithm.  This is a consistent hash
algorithm, so minimum sub files need to relocate
during directory restripe.
.RE
.P
.TP
.B NOTE
Only the root user can migrate directories.  Files that have been archived by
HSM or are currently opened will fail to migrate, user can run the same migrate
command again to finish migration when files are ready.  Both inode and
directory entry will be migrated.  During migration directory and sub files can
be accessed like normal ones, but the migration itself cannot be interrupted.
.TP
.B NOTE
It is not currently possible to migrate files with an
.B mdt
component (Data-on-MDT, DoM).  If it is necessary to migrate such files off
a particular MDT, they must first be migrated to have a non-DoM file layout
and then the inodes migrated separately.  See
.B EXAMPLES
for details on how to migrate DoM files between MDTs.
.TP
.B WARNING
Each migrated file or directory will have a new FID, and hence a new inode
number.  As a consequence, files archived by Lustre HSM that depend on
the FID as the identifier in the HSM archive cannot currently be migrated.
Having a new inode number may also cause backup tools to consider the
migrated file(s) to be a new, and cause them to be backed up again.
.P
.SH EXAMPLES
.TP
.B $ lfs migrate -c 2 /mnt/lustre/file1
This migrates the data in
.B file1
into a new layout with 2 stripes.
.TP
.B $ lfs migrate -E 256M -c 1 -E 16G -c 4 -E eof -c 40 /mnt/lustre/file2
.br
This migrates the data in
.B file2
into a three component composite layout (number of stripes depends on
file size).
.TP
.B # lfs migrate -m 0,2 testremote
.br
Recursively move the subdirectories and inodes contained in directory
.B remotedir
from its current MDT to MDT0000 and MDT0002.  The
.B testremote
directory and all of its subdirectories will be striped across both MDTs.
.TP
.B $ lfs migrate -m 0,2 -d ./testremote
Move ./testremote and the first level of sub files from their current MDT
to the MDT with index 0 and 2. Different from above case, the layout of
subdirectories under ./testremote won't be changed.
.TP
.B # lfs setstripe -E 256M -c 1 -E 16G -c 4 -E eof -c 40 topdir
Set a default PFL layout (without any DoM component) on the directory
.BR topdir ,
.TP
.B # lfs find dir -type f -L mdt -0 | xargs -0 lfs migrate --copy topdir
then find and migrate all regular files that have an
.B mdt
component to copy the default layout from the specified
.BR topdir ,
.TP
.B # lfs migrate -m 2 topdir
.br
and finally migrate the directory
.B topdir
and all files and subdirectories in that tree to MDT0002.  This allows
migrating files with DoM components off an MDT.
.SH AUTHOR
The lfs command is part of the Lustre filesystem.
.SH SEE ALSO
.BR lfs (1),
.BR lfs-setstripe (1),
.BR lfs-setdirstripe (1),
.BR lfs-getdirstripe (1),
.BR lfs-mkdir (1),
.BR lfs_migrate (1),
.BR lctl (8),
